Physiologic loading generates a host of physical forces which force prevents bone resorption are not well understood. In the past funding period we proved that an exogenous electrical field similar to that generated during skeletal loading is capable of inhibiting osteoclastogenesis. We have shown that two other load-generated forces inhigit osteoclast recruitment as well. Continuous application of 1.37 (or 2 atm) of pressure inhibits osteoclast formation by 33+5%. Mechanical strain created by stretching cells inhibits osteoclast formation by 60+5%. This work suggested a paradigm - that force limits bone resorptio by decreasing osteoclast numbers. We have begun to develop information regarding the cellular and molecular targets of force during osteoclast recruitment. Both pressure and stretch are effective if dosed during the period of culture when osteoclast precusor proliferation and erly differentiation occur. Macrophage colony stimulating factor (MCSF) is crtical for these early events in culture. Using RT- PCR we have shown that presure and stretch decrease the level of mRNA encoding the membrane-bound isoform of MCSF, which is expressed by osteoblasts. Since both osteoblasts and MCSF are necessary for osteoclastogenesis, it is possible that a force-induced decrease in osteoblast expression of membrane bound MCSF could be responsible for decreased osteoclast formation. We therefore propose that biiphysical stimuli inhibit osteoclast recruitment through modulation of osteoblast paracrine signals. We postulate that in limiting bone resorption, biopphysical forces directly target the osteoblast. The osteoblast responds to force by signalling for either decreased proliferation of osteoclast precursors or decreased entry of these precursors into the osteoclast lineage. These effects may be transduced by decreases in osteoblast MCSF expression. To elucidate the mechanisms of force in this system we will 1) quantify an inhibitory dose-response to pressure and stretch and assign a minimal window of "force sensitivity" to limit possible targets of force in time. 2) We will define which cell is the critical direct target of effective forces - the osteoclast precursors and/or osteoblast precursor. 3) We will then clarify whether the distal target of force is the proliferation or eraly differentiation of osteoclast precursors. Finally, we hope to 4) distinguish at least one molecular mechanism involved in this process, and will pursue the idea that force induced decrements in membrane bound MCSF lead to an inhibition of osteoclast formation.